The present invention relates to a DC-DC converter for supplying a DC voltage to various electronic apparatuses, and more particularly, to a DC-DC converter capable of raising a voltage using a switching system.
In recent years, step-up circuits in switching-type DC-DC converters are used as step-up circuits for numerous electronic apparatuses in which a battery is used as a DC power supply since the step-up circuits have high power conversion efficiency. FIG. 5 is a circuit diagram showing the configuration of a conventional step-up circuit disclosed in Japanese Patent Application Laid-open No. Sho 61-92165. Generally speaking, a step-up circuit is configured as shown in FIG. 5. That is to say, one terminal of an inductor 202 is connected to a DC power supply 201, one terminal of a main switch 203 is grounded, and the other terminal thereof and one terminal (anode) of a diode 204 are connected to the other terminal of the inductor 202. To the other terminal (cathode) of the diode 204, an output capacitor 205 is connected, and an output voltage Vo is supplied to a load 206.
The output voltage Vo is input to an error amplifier 207, compared with a reference voltage Vref and fed back to a control circuit 208. The control circuit 208 controls the ON/OFF operation of the main switch 203 so that the output voltage Vo is stabilized at a target value. By the ON/OFF operation of the main switch 203, energy storing and discharging in the inductor 202 are repeated, and the output voltage Vo higher than the input voltage Vi of the DC power supply 201 is generated. In the conventional step-up circuit configured as described above, when the diode 204 breaks down and short-circuited, the electric charge charged in the output capacitor 205 flows reversely to the main switch 203 via the short-circuited diode 204 and is discharged. At the time of the discharging, there is a danger that the main switch 203 may also be damaged.
To protect the main switch 203 at the time when the diode 204 is short-circuited as described above, a protection circuit is provided for the step-up circuit shown in FIG. 5. Referring to FIG. 5, numeral 230 designates a current detector for detecting the current flowing in the main switch 203, numeral 231 designates a comparator for judging whether the current flowing in the main switch 203 has reached a predetermined level or not, numeral 240 designates a current detector for detecting the current flowing in the diode 204, numeral 241 designates a comparator for judging whether the current flowing reversely to the diode 204 has reached a predetermined level or not, numeral 242 designates an OR circuit for outputting the logical OR of the outputs of the comparator 231 and the comparator 241 to the control circuit 208.
In the case that the diode 204 is short-circuited, when the main switch 203 turns ON, the current flowing reversely in the diode 204 increases abruptly, and the output of the comparator 241 becomes H level. Hence, the OR circuit 242 outputs an H-level signal to the control circuit 208. The control circuit 208, to which the H-level signal is input from the OR circuit 242, turns OFF the main switch 203 to protect the main switch 203 against overcurrent.
It is ideal that a diode is configured so that no current flows reversely during the ordinary operation. Hence, in the protection circuit 208 configured as described above, the reverse current detection level detected using the current detector 240 and the comparator 241 can be set so as to be lower than the maximum turn OFF current level of the main switch 203 detected using the current detector 230 and the comparator 231. For this reason, the reverse current can be detected accurately, and the main switch 203 can be protected securely.
However, in the step-up circuit in the conventional DC-DC converter configured as described above, the current flowing in the main switch 203 is detected, and the current flowing in the diode 204 is also detected. In addition, although it is preferable that a synchronous rectification circuit should be used instead of the diode 204 to raise response, the synchronous rectification circuit allows reverse current flow even during the ordinary operation in some cases, and in such a case, the reverse current detection level must be set high. It is thus difficult to apply the above-mentioned protection circuit.
Furthermore, as portable apparatuses are made compact in recent years, switching devices, such as a main switch, and rectifiers, such as a diode, being used for a step-up circuit in a DC-DC converter are frequently mounted in a one-chip semiconductor integrated circuit. In such a semiconductor integrated circuit, it is desirable that the components thereof are not broken down when a short-circuit test is carried out between adjacent terminals. In other words, during such a short-circuit test, the rectifier is short-circuited in some cases, and during the short-circuit test, the other components must be prevented from being broken down at any cost. Since the short-circuited state of the rectifier occurs not only owing to the breakdown of the rectifier itself but also owing to other causes, after such causes for the short-circuited state are eliminated, it is desired that the ordinary operation is restored automatically.